1. Field of the Invention
The present invention relates to an optical glass and optical elements comprising the optical glass.
2. Description of Related Art
Glasses in the lanthanum crown and lanthanum flint region of the Abbe diagram having a high refractive index and a high Abbe number are sought after, in particular, for the fields of application of imaging, sensor technology, microscopy, medical technology, digital projection, photolithography, laser technology, wafer/chip technology and also for telecommunications, optical messaging technology and optics/illumination in the automotive sector.
Recently, instead of the hitherto customary cutting out of optical components from block or ingot glass, production processes in which directly pressed bodies, i.e. blank-pressed (precision pressed) optical components, and/or close-to-final-shape preforms or intermediate bodies for further pressing, known as “precision gobs”, can be obtained directly subsequent to melting of the glass. In general, “precision gobs” are considered to be preferably fully fire-polished, semifreely or freely shaped glass portions which have the same mass and a very similar shape to the desired optical components.
For this reason, increased demand for “short” glasses, i.e. glasses whose viscosity varies greatly with a change in temperature, has recently been reported for process technology in the melt and hotforming. In the production process, this viscosity behaviour has the advantage that the hotforming times and thus the mould closure times can be reduced. As a result, firstly the throughput is increased. Secondly, the moulding material is spared in the close-to-final-geometry precision hotforming. Thus, the overall production costs are reduced. In addition, the more rapid solidification makes it possible to process short glasses having a relatively great tendency to crystallize, which is not possible in the case of similarly crystallization-sensitive longer glasses. In addition, prenucleation which could be a problem in subsequent secondary hotforming steps is avoided or at least drastically reduced.
To achieve the desired optical properties, in particular the low dispersion, i.e. a high Abbe number, conventional optical glasses of this optical position generally contain PbO. In addition, As2O3 was frequently used as refining agent. Since the glass components PbO and As2O3 have been considered to be environmentally problematical in recent years, lead- and arsenic-free glasses are preferably being used by most producers of optical instruments and products. In addition, PbO-containing glasses have poor chemical resistance. However, glasses having improved chemical resistance are always of importance for use in high-grade products.
Known lead-free glass variants of the same optical position often contain large amounts of TiO2 in a siliceous glass matrix. However, this gives extremely crystallization susceptible glasses which can frequently not be processed in a secondary hotforming step. Owing to their great hardness, these glasses are also very difficult to work mechanically. Furthermore, such glasses have an impaired transmission at the “blue edge” of the transmission spectrum, i.e. at a wavelength of <420 nm.
As further alternatives, glasses of this optical position have recently frequently been obtained by the use of Bi2O3. However, these glasses have the disadvantage that they are very sensitive to the redox conditions in the melting apparatus and under unfavourable redox conditions there is the risk of a reduction in transmission due to the formation of Bi0.
It is therefore an object of the present invention to provide optical glasses which avoid the above mentioned disadvantages of the prior art and to achieve the desired optical properties. In particular, a composition range for short optical glasses which makes the desired optical properties (nd/νd) possible is to be found. The glasses should have a high transmission, good chemical resistance and workability, low production costs and good environmental friendliness. These glasses should preferably be able to be processed via the blank pressing process (precision pressing process) and therefore have low transformation temperatures Tg. Furthermore, they should be able to be readily melted and processed and also have a sufficient crystallization stability for a secondary hotforming step and/or for manufacture in continuously operated apparatuses. Furthermore, a very short glass in a viscosity range from 107.6 to 1013 dPas is desirable.